Almost any animal in its right mind will flee a burning forest, but there is one insect that does the opposite. The jewel beetle Melanophila just loves a good fire, because its larvae can develop only in the wood of freshly burnt trees. Biologists have long known that Melanophila home in on fire with special infrared receptors. But in today's issue of Nature, German biologists report that the beetle has an additional device: a highly sensitive smoke detector up its antenna.
Stefan Schütz of Justus Liebig University in Giessen and his colleagues pulled out antennae from Melanophila beetles and hooked them up to a gas chromatograph, a machine that can disentangle complex mixtures of chemicals. They injected fumes from smoldering pinewood into the machine and watched which compounds an antenna would respond to, as shown by tiny electrical currents in its nerves. They discovered that the beetles' antennae were most receptive to a substance called guaiacol, released by burning wood. As little as a few parts per billion managed to excite Melanophila. Some calculations show that this should allow the insect to find a single smoldering tree from a kilometer away, says Schütz.
George Evans, an ecologist from the University of Alberta in Edmonton, Canada, who discovered the insect's infrared detectors back in the 1960s, says he's not surprised that Melanophila can also smell smoke. However, he thinks the bug's infrared sensors are probably much more powerful than its smoke detectors; he recalls seeing beetles in Alberta being attracted to fires, lighted to clean oil pipes, that were up to 80 kilometers away. "The beetles would come in there by the thousands and they wouldn't know what to do next, so they'd just sit there," he says. The fires were too far for the smoke to have wafted, and besides, oil smoke doesn't contain guaiacol, says Evans, so the insects must have gone after the infrared.
Schütz says the two sensory systems probably allow the beetles to use multiple strategies: In open areas, where they have a clear line of sight, they will rely on infrared sensors, while their nose for smoke may help them in more hilly habitats.